This is an application for a renewal of a NIDA sponsored K02 Independent Scientist Award. G protein-coupled receptors (GPCRs) comprise the largest known family of signaling receptors, mediate diverse functions in the CNS, are critical targets for drugs of abuse, and are extensively regulated in vivo. One mechanism of receptor regulation, on which clinically relevant drugs can have diverse effects, involves regulated membrane trafficking of receptors through the endocytic pathway. In previous studies a specific mechanism by which opioid neuropeptide receptors undergo initial endocytosis from the plasma membrane was identified and investigated. The current proposal seeks to elucidate mechanisms by which the endocytic membrane trafficking of G protein-coupled opioid receptors is controlled after endocytosis. The Specific Aims of the proposed studies are to: (1) Define the biochemical properties of a mechanism that mediates signal-dependent recycling of opioid receptors; (2) Elucidate a distinct mechanism that promotes sorting of endocvtosed opioid receptors to lysosomes; (3) Determine whether opioid receptors are sorted between distinct membrane domains of multivesicular endosomes; and (4) Investigate the functional relevance of specific post-endocytic sorting mechanisms to opioid receptor regulation in neurons. These studies have general relevance to understanding mechanisms by which G protein-coupled receptors are regulated, and will contribute fundamental information to understanding physiological adaptation of the nervous system in response to clinically important opiate drugs.